Transition-metal-based supramolecules have been extensively explored in recent years. These supramolecules, which can be squares, helices, boxes, cages, or rectangles, are of particular interest due to their applications in host-guest interactions, molecular recognition, and as molecular sensors. See, e.g., Lehn, Supramolecular Chemistry: Concepts and Perspectives; VCH: Weinheim, 1995; Leininger, et al. (2000) Chem. Rev. 100: 853; Swiegers & Malefetse (2000) Chem. Rev. 100: 3483; Caulder & Raymond (1999) J. Chem. Soc., Dalton Trans. 1185; MacGillivray & Atwood (1999). Angew. Chem. Int. Ed. 38: 1018; Constable & Schofield (1998) Chem. Commun. 403; Fujita (1999) Acc. Chem. Res. 32: 53; Fujita (1998) Chem. Soc. Rev. 27: 417; Imamura & Fukushima (2000) Coord. Chem. Rev. 198: 133; Dixon et al. (2000) Inorg. Chem. 39: 3432; and Sun & Lees (2000) J. Am. Chem. Soc. 122: 8956.
It has been established that a transition-metal-based supramolecule offers the feasibility of host-guest interactions and molecular recognition phenomena based on Coulombic or hydrophobic binding, where the binding affinity is highly dependent on the cavity size inside the supramolecule (Sun & Lee supra). The cavity may function as a catalytic microreactor. The supramolecule has been known to possess a net charge and require a counterion within its cavity. The counterion affects the cavity size as well as molecular sensing properties of the supramolecule. It is desirable to design and prepare a supramolecule that is neutral.
In addition, incorporation of photo-active centers into a transition-metal-based supramolecule is also desirable in molecular sensing technology. This application provides an alternative to the detection of guest inclusion based on photoluminescence characteristics. For example, if a supramolecule is luminescent, it can be used as a tool in lieu of 1H NMR spectroscopy for detecting guest inclusion and studying electronic excited state reactivity and possible manipulation of reactivity by encapsulated guests. See, e.g., Slone et al. (1998) Coord. Chem. Rev. 171: 221; De Silva et al. (1997) Chem. Rev. 97: 1515; and Keefe et al. (2000) Coord. Chem. Rev. 205: 201.
There is a need for designing and preparing a transition-metal-based supramolecule that is neutral and luminescent.